Bump Maps are missing after exporting as glTF 2.0
The glTF format uses normal maps, not bump maps1. If you can bake your bump map into a normal map it should export correctly and appear the same. Sketchfab also allows you to download your model as glTF, so it may be worth trying that.



1The term "bump map" can mean different things, so I'm generalizing here... For a more precise answer I might need to see your material setup in Blender.